Electromechanical actuators, and particularly electromechanical motors, have been widely applied for many different tasks during recent years. High force, small size, high speed, high-precision positioning and inexpensive manufacturing are attractive characteristics of many of the prior-art motors. However, the attractive characteristics are often contradictory, and optimizing regarding one aspect often reduces other qualities.
Electromechanical actuator arrangements using drive elements presenting a two-dimensional motion have been discussed for awhile. In U.S. Pat. No. 5,345,137, a two-dimensional driving ultrasonic motor is disclosed. By laminating electromechanical volumes with electrodes having different geometrical extensions, the entire stack can be controlled to bend in different directions.
In the article by S. Johansson, “One approach towards the fabrication of a microrobot system”, Material Science and Engineering C2 (1995), pp. 141–149, an actuator comprising six hexagonal elements soldered onto a common actuator backing is proposed. The hexagonal elements have an electrode structure, which enables the top of the elements to move in a controlled manner in three dimensions. A stepping mechanism can be obtained, presenting extremely high precision. In the closely related patent U.S. Pat. No. 6,184,609, the arrangement is improved by having the actuator backing and the elements manufactured as one integrated monolithic block. Such arrangement further improved the positioning precision using a non-resonant repetition of small steps. However, the speed and power efficiency was somewhat limited.
In DE 4408618, a common base connects two sets of bimorph drive elements to each other. The drive elements and the base are cut out from a common piezoceramic plate. The drive elements are driven by phase-shifted sinusoidal voltages giving rise to elliptical motion paths of the tips of the elements. A similar approach is used in U.S. Pat. No. 6,066,911. Stacks of piezoelectric layers are formed side by side on a common piezoelectric base. This drive element was intended to be driven in the ultrasonic frequency range, and thereby benefit from high speed and high power efficiency. However, estimations of operation conditions based on the information given in this disclosure reveals that the drive element would be difficult to operate for any longer periods of time. The excitation and de-excitation of the piezoceramic material develops a lot of heat. Driving the disclosed drive element at the proposed conditions would within a very short time lead to an extensive heating of the drive elements. Such heating typically leads to unpredictable remaining deformations of the drive elements and the actuator backing, weakening of soldered joints and if the Curie temperature of the drive elements is exceeded, the polarization of the drive elements will disappear.
In U.S. Pat. No. 6,337,532, a similar basic approach is used, but the operation is intended for a non-resonant walking operation. The excitation of the piezo-legs is performed in a very controlled manner, giving a smooth motion and a very accurate positioning. However, the operation frequencies are far below resonance.